Ozone is generally produced on a commercial scale by subjecting substantially pure oxygen to a high voltage discharge, which causes some of the oxygen to be converted to ozone. Conventional ozone generators produce a product stream containing about 10% by weight ozone, which is satisfactory for many ozone applications. However, because the efficiency of some industrial ozone-based processes, such as waste water treatment and pulp and paper bleaching operations, is directly proportional to the concentration of ozone in the treatment gas fed to the processes, there is a demand for equipment that can produce ozone product gases which contain higher ozone concentrations than do currently available ozone gas products. In response to this demand, equipment manufacturers have made improvements in ozone generators which make it possible to make ozone product gas containing up to about 14% by weight ozone. However, the improved ozone generators are considerably more costly to operate than are earlier ozone generators, since the improved generators consume significantly more power than do the earlier generators.
Waste water treatment and paper and pulp plant operators would like to have available ozone generating equipment that can produce ozone gas products containing up to 20% by weight or more ozone, but equipment having such capability is not currently available. Furthermore, even if such equipment were available, it would be prohibitively expensive to operate because of the very high power consumption that would be required to produce ozone gas of this quality.
Because of the continuing need for product gas containing high concentrations of ozone, various techniques for increasing the ozone concentration of ozone-oxygen gas mixtures by separating ozone from oxygen have been considered. One procedure that has been investigated is distillation. Since ozone has a boiling point of about -112.degree. C. and oxygen has a boiling point of about -190.degree. C., distillation would appear to be an attractive method for separating these gases. U.S. Pat. No. 5,756,054 discloses an ozone generating system in which liquid oxygen from a cryogenic oxygen source is subjected to ozonization to produce an ozone-containing product gas, ozone is separated from the product gas by condensation and the gaseous oxygen fraction is recycled to the cryogenic oxygen source. A major drawback of this method of separation is that it is capital- and energy-intensive.
Another ozone-oxygen separation technique that has been explored is adsorption. Ozone is generally more strongly adsorbed by adsorbent materials than is oxygen and thus it can be readily separated from oxygen by adsorption-based processes. U.S. Pat. No. 5,507,957 discloses an ozone generating system in which oxygen is separated from air in an adsorption vessel and the separated oxygen is subjected to ozonization to produce an ozone-containing stream, which is recycled to the adsorption vessel. Ozone in the recycle stream is adsorbed by a preliminary bed of adsorbent and the oxygen contained in the recycle stream passes through the preliminary adsorbent and is recycled to the ozonizer. A problem associated with the use of adsorption for the separation of ozone and oxygen is that the sorbed ozone component cannot be recovered from the adsorption equipment until the adsorbent regeneration phase of the separation process. As was the case with distillation, it is difficult or impossible to recover the ozone product stream from the adsorption equipment without appreciable decomposition of the ozone. A further complication of ozone-oxygen adsorptive separation processes is the fact that some adsorbents actually catalyze the decomposition of ozone.
Membranes have been investigated for nondispersively introducing ozone into water streams from ozone-oxygen gas mixtures. Shanbhag et al., "Membrane-Based Ozonization of Organic Compounds", Ind. Eng. Chem. Res., vol. 37, 1998, pp. 4388-4398, describes the ozonation of water which contains organic pollutants by contacting a silicone membrane with an ozone-oxygen gas mixture. Ozone from the gas mixture passes through the membrane and contacts the pollutant-containing water on the permeate side of the membrane.
The present invention provides an efficient and effective method of increasing the ozone concentration of ozone-oxygen gas without significant loss of ozone by decomposition. This result is accomplished by separating an ozone-enriched gas stream from an ozone-oxygen gas mixture using a highly ozone-selective membrane manufactured from an elastomeric polymer or silica.